Flow-guiding plate for a fuel cell

ABSTRACT

A flow-guiding plate for a fuel cell, including a conductive sheet including a relief: defining alternating flow channels on first and second faces, two successive channels on the first face being separated by walls; defining first and second access holes at ends of each of the flow channels on the second face and of a first group of flow channels on the first face; defining a flow restriction in each flow channel of a second group of flow channels on the first face, the cross-section of the flow restrictions being smaller than the cross-section of the access holes to the flow channels of the first group, the first face including alternating flow channels of the first group and alternating flow channels of the second group.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/110,127, filed Jul. 7, 2016, which claims priority from InternationalApplication No. PCT/FR2015/050013, filed Jan. 6, 2015, which is basedupon and claims the benefit of priority from French Patent Application1450065, filed Jan. 1, 2017, the entire contents of each of which areincorporated herein by reference.

BRIEF SUMMARY

The invention relates to fuel cells and in particular to fuel cellscomprising an alternation of proton exchange membranes and bipolarplates.

Fuel cells are envisaged as electrical supply systems for motor vehiclesproduced on a large scale in the future, and also for a large number ofapplications. A fuel cell is an electrochemical device which convertschemical energy directly into electrical energy. A fuel such asmolecular hydrogen or methanol is used as fuel for the fuel cell.

In the case of molecular hydrogen, the latter is oxidized and ionized onan electrode of the fuel cell and an oxidant or combustion agent isreduced on another electrode of the fuel cell. The chemical reactionproduces water at the cathode, oxygen being reduced and reacting withthe protons. The great advantage of the fuel cell is that it avoidsdischarges of atmospheric polluting compounds on the site of electricitygeneration.

Proton exchange membrane fuel cells, known as PEM fuel cells, operate atlow temperature and exhibit particularly advantageous compactnessproperties. Each cell comprises an electrolytic membrane which allowsonly the passage of protons and not the passage of electrons. Themembrane comprises an anode on a first face and a cathode on a secondface, in order to form a membrane electrode assembly, known as an MEA.

At the anode, the molecular hydrogen is ionized to produce protons whichpass through the membrane. The electrons produced by this reactionmigrate to a flow plate and then pass through an electric circuitexternal to the cell in order to form an electric current. At thecathode, oxygen is reduced and reacts with the protons to form water.

The fuel cell can comprise several plates, known as bipolar plates, forexample made of metal, stacked on one another. The membrane ispositioned between two bipolar plates. The bipolar plates can compriseflow channels and orifices for guiding the reactants and the productstoward/from the membrane, for guiding cooling liquid and for separatingdifferent compartments. The bipolar plates are also electricallyconducting in order to form collectors of the electrons generated at theanode. The bipolar plates also have a mechanical role of transmittingthe strains of clamping of the stack necessary for the quality of theelectrical contact. Gas diffusion layers are interposed between theelectrodes and the bipolar plates and are in contact with the bipolarplates.

Electron conduction is carried out through the bipolar plates, ionconduction being obtained through the membrane.

The bipolar plates continuously supply the reactive surfaces of theelectrodes with reactants, as they are consumed. The distribution of thereactants at the electrodes has to be as homogeneous as possible overthe whole of their surface. The bipolar plates comprise networks of flowchannels which provide for the distribution of the reactants. A networkof flow channels is dedicated to the anode fluid and a network of flowchannels is dedicated to the cathode fluid. The networks of anode andcathode flow channels are never in communication inside the fuel cell,in order to prevent direct combustion of the fuel and the oxidant. Thereaction products and the unreactive entities are discharged byentrainment by the flow as far as the outlet of the networks ofdistribution channels. In the majority of the architectures encountered,the bipolar plates comprise flow channels traversed by cooling liquid,making possible the discharge of the heat produced.

Three forms of circulation of the reactants in the flow channels aremainly distinguished:

-   -   serpentine channels: one or more channels run through the entire        active surface in several to-and-from paths.    -   parallel channels: a bundle of parallel and traversing channels        runs right through the active surface,    -   interdigital channels: a bundle of parallel and blocked channels        runs right through the active surface. Each channel is blocked,        either on the side of the fluid inlet or on the side of the        fluid outlet. The fluid entering a channel is then forced to        pass locally through the gas diffusion layer in order to join an        adjacent channel and subsequently reach the fluid outlet of this        adjacent channel.

The flow channels can be straight or slightly wavy.

The materials most commonly used for the bipolar plates arecarbon-polymer composite and embossed metal.

The embossed metal proves to be a solution which favors the lighteningand the compactness of the fuel cells. The bipolar plates then use thinmetal sheets, for example made of stainless steel. The flow channels areobtained by embossing. Most frequently, use is made of a first flowplate in the form of a first embossed sheet defining the anode flowchannel and of a second flow plate in the form of a second embossedsheet defining the cathode flow channel. These two sheets of the flowplates are assembled back to back by welding to form a bipolar plate. Aflow channel for the cooling fluid is put into the space between thesheets.

The carbon-polymer composite technology makes possible greaterflexibility in design of the flow channels by molding thicker plates.

The document FR 2 973 583 provides for the deletion of the coolingliquid channel in one bipolar plate out of two. With bipolar plates madeof sheet metal, the bipolar plate devoid of a cooling liquid channelcomprises just one embossed sheet, which lightens the fuel cell. Thecathode flow channels are formed on a first face of the sheet, while theanode flow channels are formed on the other face of the sheet.

The design of these channels is then closely related, since the anodeface is the negative of the cathode face. Nevertheless, the pattern ofthe channels has to guarantee that the flows in the bipolar plateshaving just one sheet are similar to those in the bipolar plates havingtwo sheets, in order not to create an imbalance in supply between thedifferent cells. Furthermore, it is desirable for one and the same sheetto be able to be used without distinction for a bipolar plate having asingle sheet or to form a bipolar plate having two sheets.

An additional design constraint relates to the drops in pressure in theflows of reactants, these drops in pressure having to have one and thesame order of magnitude at the anode and at the cathode. This constrainthas an effect on the respective cross sections of the fuel and oxidantflow channels. This constraint complicates the design of a bipolar platehaving just one sheet.

For molecular hydrogen used as fuel, the passage cross section in theanode channels has to be smaller than that in the cathode channels inorder to obtain a drop in pressure of the same order of magnitude. Thisis because molecular hydrogen is less viscous than the oxidantcirculating at the cathode and its flow rate is lower.

The molar flow rate of molecular hydrogen consumed in a cell is equal toI/, I being the electric current produced and F the Faraday constant.The molar flow rate of air consumed is, for its part, equal to 1.2*I/F.

In practice, the flow rates of fuel and oxidant introduced into thecells are always greater than the flow rates consumed, according to anovervaluation or mark-up factor. For molecular hydrogen, theovervaluation factor is generally between 1 and 2.5. For air, theovervaluation factor is generally between 1.2 and 3, in order toguarantee a sufficient amount of oxygen at the outlet. Thus, for a givencurrent I, the ratio of the molar flow rate of air to the molar flowrate of molecular hydrogen is at least equal to 2 and most commonlybetween 3 and 5.

The viscosity of humid molecular hydrogen is of the order of 8*10⁻⁶ to13*10⁻⁶ Pa·s, depending on the temperature and the moisture content. Theviscosity of humid air is of the order of 12*10⁻⁶ to 21*10⁻⁶ Pa·s.

When the flow channels of a bipolar plate are identical on the hydrogenand air sides, a ratio of the drop in pressure for the air to the dropin pressure for the molecular hydrogen of between 2 and 10 is obtained.In order to balance the drops in pressure, it is desirable to reduce thepassage cross section of the molecular hydrogen flow channels with aratio of between 2 and 10 with respect to the passage cross section ofthe air flow channels.

Several alternatives are known for reducing this disproportion in dropsin pressure, with associated disadvantages:

-   -   increasing the width of the cathode flow channels, producing        anode channels having the thinnest width which it is        industrially possible to produce. This contradicts research on        the optimum effectiveness of electron conduction in the fuel        cell, which is generally obtained by reducing as much as        possible the width of the flow channels;    -   producing fewer flow channels at the anode than at the cathode.        The distance between two anode channels increases, as well as        the width of the teeth separating the channels. This structure        is unsuitable for a bipolar plate having just one sheet as the        electron conduction from the cathodes is then greatly affected;    -   decreasing the depth of the anode flow channels. This structure        is unsuitable for a bipolar plate having just one sheet as the        anode and cathode channels automatically have one and the same        depth and are thus affected by this decrease in depth.

BRIEF SUMMARY

The invention is targeted at overcoming one or more of thesedisadvantages. The invention is targeted in particular at makingpossible the use of identical sheets for a bipolar plate having a singleflow plate and for a bipolar plate having double flow plates, withoutanode and cathode flow disparities for these two types of bipolarplates, and while promoting uniformity in the drops in pressure betweenthe anode channels and the cathode channels. The invention thus relatesto a flow guiding plate for a fuel cell.

The invention additionally relates to a fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will clearlyemerge from the description which is given thereof below, by way ofindication and without any limitation, with reference to the appendeddrawings, in which:

FIG. 1 is a view in exploded perspective of an example of a fuel cell;

FIG. 2 is a view in perspective of flow channels on a first face of afirst embodiment of a metal sheet for a fuel cell flow plate;

FIG. 3 is a top view of the first face of the metal sheet of FIG. 2;

FIG. 4 is a view in cross section of the metal sheet of FIG. 2 atorifices for access to flow channels;

FIG. 5 is a view in cross section of the metal sheet of FIG. 2 in itsmedian part;

FIG. 6 is a view in cross section of the metal sheet of FIG. 2 at asecond end of the flow channels;

FIGS. 7 and 8 are views in perspective of the second face of the sheetof FIG. 2 at orifices for access to the flow channels;

FIG. 9 is a view in cross section of a fuel cell including differentbipolar plates formed from metal sheets of FIG. 2;

FIGS. 10 to 13 illustrate different views in longitudinal section of thefuel cell of FIG. 9;

FIGS. 14 and 15 are views in perspective of the first face of second andthird embodiments of metal sheets;

FIG. 16 is a view in cross section of an alternative form of the firstembodiment of a metal sheet at orifices for access to the flow channels.

DETAILED DESCRIPTION

FIG. 1 is a schematic view in exploded perspective of a stack of cells 1of a fuel cell 4. The fuel cell 4 comprises several superposed cells 1.The cells 1 are of the type having a proton exchange membrane or polymerelectrolyte membrane.

The fuel cell 4 comprises a source of fuel 40. The source of fuel 40 inthis instance supplies an inlet of each cell 1 with molecular hydrogen.The fuel cell 4 also comprises a source of oxidant 42. The source ofoxidant 42 in this instance supplies an inlet of each cell 1 with air,the oxygen of the air being used as oxidant. Each cell 1 also comprisesescape channels. Each cell 1 also exhibits a cooling circuit.

Each cell 1 comprises a membrane electrode assembly 110 or MEA 110. Amembrane electrode assembly 110 comprises an electrolyte 113, a cathode112 (not illustrated in FIG. 1) and an anode 111 which are placed oneither side of the electrolyte and attached to this electrolyte 113.

A bipolar plate is positioned between each pair of adjacent MEAs. Thefuel cell 4 in this instance comprises an alternation of bipolar plates51 and 52.

The bipolar plates 51 include a flow guiding plate 53 formed of a singlemetal sheet [[53]]. In a bipolar plate 51, the relief of a first face ofits metal sheet defines the anode flow channels, and the relief of asecond face of its metal sheet defines the cathode flow channels.

The bipolar plates 52 include two flow guiding plates 53. Each of theseflow guiding plates 53 includes one metal sheet. The sheets aresuperimposed and attached together via welds 54. In a bipolar plate 52,the relief of one face of a first flow guiding plate 53 defines theanode flow channels and the relief of one face of a flow guiding plate53 defines the cathode flow channels. A cooling liquid flow channel isformed in between the metal sheets of the flow guiding plates 53 of abipolar plate 52.

The metal sheets forming the flow guiding plates 53 of a bipolar plate52 are identical. The metal sheets forming the flow guiding plates 53 ofthe bipolar plates 51 and 52 are identical. Thus, one and the samedesign and one and the same manufacturing process can be used for themanufacture of the metal sheets of the flow guiding plates 53 of thebipolar plates 51 and 52. The metal sheets are, for example, formed ofstainless steel, of steel alloy, of titanium alloy, of aluminum alloy,of nickel alloy or of tantalum alloy.

In a way known per se, during the operation of the cell 1:

-   -   molecular hydrogen flows in an anode flow pipe between a bipolar        plate and an anode 111;    -   air flows in a cathode flow pipe between a bipolar plate and a        cathode 112.

At the anode 111, the molecular hydrogen is ionized to produce protonswhich pass through the MEA 110. The electrons produced by this reactionare collected by the bipolar plate positioned facing this anode 111. Theelectrons produced are subsequently applied to an electric chargeconnected to the fuel cell 4 in order to form an electric current. Atthe cathode 112, oxygen is reduced and reacts with the protons to formwater. The reactions at the anode and the cathode are governed asfollows:

H₂→2H⁺+2e ⁻ at the anode;

4H⁺+4e ⁻+O₂→2H₂O at the cathode.

During its operation, a cell of the fuel cell normally generates acontinuous voltage between its anode and its cathode of the order of 1V. The catalyst material used in the anode 111 advantageously includesplatinum, for its excellent catalytic performance.

FIG. 2 is a view in perspective of flow channels on the anode side of afirst embodiment of a conducting sheet of a flow guiding plate 53according to the invention. FIG. 3 is a top view of the flow guidingplate 53. Such a flow guiding plate 53 can be used to form differenttypes of bipolar plates, as described in detail subsequently.

The flow guiding plate 53 comprises a first face 55, illustrated in FIG.2, and a second face 56. The relief of the flow guiding plate 53 definesan alternation of flow channels on the opposite faces 55 and 56. Thus,two successive flow channels of one and the same face of the flowguiding plate 53 are separated by walls 559 for delimitation of a flowchannel of the other face. The flow channels of the faces 55 and 56extend along one and the same longitudinal direction. In this example,the flow channels are substantially straight.

At the face 55, the flow channels are divided into first and secondgroups.

Access orifices 551 emerge in flow channels 553 of the first group. Theaccess orifices 551 are positioned at a first end of the flow channels553. A first flat part 535 forms a flow guiding surface extendingbetween the different access orifices 551. A sloping wall 557 forms ajunction between an access orifice 551 and the median part of its flowchannel 553.

At the second end of the flow channels 553, access orifices 552 emergein the flow channels 553 of the first group. A second flat part 536forms a flow guiding surface extending between the different accessorifices 552. A sloping wall 558 forms a junction between the accessorifices 552 and the median part of their flow channel 553.

The cross section of each flow channel 553 in its median part, betweenits access orifices 551 and 552, is greater than the cross section atthe access orifices 551 and 552. The sloping walls 557 and 558 thusextend downward as far as a flat bottom of the flow channel 553. Themedian part of the flow channels 553 is thus devoid of flowrestrictions.

In order to guarantee uniform drops in pressure in the different flowchannels 553, the latter exhibit one and the same cross section in theirmedian part.

The face 55 exhibits flow channels 554 of the second group. The face 55comprises an alternation of flow channels 553 and 554 along a transversedirection. In the embodiment illustrated in FIG. 2, the flow channels554 comprise a wall 556 at their first end and a wall 555 at theirsecond end. Each wall 555 extends from the bottom of a flow channel 554as far as the top of the flat part 536. Each wall 556 extends from thebottom of a flow channel 554 as far as the top of the flat part 535.Thus, the cross section of the flow channels 554 at the walls 555 and556 is smaller than the cross section of the orifices 551 and 552 foraccess to the flow channels of the first group. Each of these walls 555and 556 thus forms a flow restriction of a flow channel 554. Each flowchannel 554 thus comprises at least one flow restriction. The passagesection of a flow restriction is defined as the cross section of theflow channel at this flow restriction. The walls 555 and 556 extend inthis instance as far as the top of their flow channel 554 and as far asthe bottom of the flow channels 563 of the face 56 which are describedin detail subsequently.

The relief of the flow guiding plate 53 delimits flow channels 563 atthe face 56. FIGS. 7 and 8 illustrate in perspective the first ends oftwo types of orifices 561 for access to the flow channels 563. Each flowchannel 563 is delimited with respect to the flow channels 553 and 554of the face 55 via walls 559. The flow channels 553 and 554 compriserespective bottom walls 565 and 566 intended to form, for example,cathode conduction contacts at the face 56.

As illustrated in these figures, the alternation of the flow channels553 and 554 on the face 55 results in two types of flow channels 563 onthe face 56. The cross section of each flow channel 563 in its medianpart, between the access orifices at its longitudinal ends, is greaterthan the cross section at these access orifices. Sloping walls thusextend downward from an access orifice as far as a flat bottom of theirflow channel 563. The median part of the flow channels 563 is thusdevoid of flow restrictions.

It is thus found that, as a result of the alternation of the flowchannels 553 and 554, the two types of flow channels 563 and of theiraccess orifices are symmetrical to one another with respect to alongitudinal plane perpendicular with respect to the mean plane of theflow guiding plate 53. Consequently, the drops in pressure through thetwo types of flow channels 563 will be absolutely identical. In order toguarantee uniform drops in pressure in the different flow channels 563,the latter exhibit one and the same cross section in their median part.

In this embodiment, two successive flow channels 563 are incommunication at each of their ends, as a result of the passages formedin order to build the walls 555 and 556 at the access orifices.Consequently, possible dispersions in drops in pressure between twosuccessive channels 563 can be rendered uniform by bringing them intocommunication in such a way.

FIG. 4 is a view in cross section of the flow guiding plate 53 in theflat part 535. FIG. 5 is a view in cross section of the flow guidingplate 53 in the median part of the flow channels. FIG. 6 is a view incross section of the flow guiding plate 53 at the access orifices 552.

The flat part 535 extends between the access orifices of the first endof the flow channels 563. The flat part 535 thus forms a distributorbetween the orifices for access to the flow channels 563 at this firstend.

The flat part 536 extends between the access orifices of the second endof the flow channels 563. The flat part 536 thus forms a distributorbetween the orifices for access to the flow channels 563 at this secondend.

The walls 559 provide separation between the flow channels of the twofaces. The bottom of a channel 563 is thus positioned at the top of achannel 553 or 554, and vice versa. The bottoms of the different flowchannels are intended to form conducting surfaces for collecting theelectric current which has to pass through the flow guiding plates 53.

The orifices for access to the flow channels 563 exhibit one and thesame section at the two ends of these flow channels. The access orifices562 of the second end of the flow channels 563 are illustrated in FIG.6. These access orifices 562 exhibit in this instance a heightcorresponding to half the height of the channels 563. The assembling oftwo flow guiding plates 53 in a stack of cells of a fuel cell 4 is thusfacilitated.

The flow guiding plate 53 is in this instance formed of a metal sheetembossed in order to confer a relief on it. The shape of the face 55 isthus the complement or the negative of the shape of the face 56. Theflow guiding plate 53 can, for example, be produced by embossing a metalsheet.

In this example, the flow channels 553 and 554 exhibit one and the samecross section over at least 75% of their median part. Thus, the bottomof the flow channels 554 provides a very large surface area forcollecting, if appropriate, a cathode current.

In this example, the flow channels 553, 554 and 563 exhibit an identicalcross section in their median part. Consequently, these channels mayexhibit the minimum width corresponding to their technology offormation, in order to optimize the uniformity in distribution of thecurrent through the bipolar plate to be formed. Furthermore, the use ofidentical cross sections for the flow channels of the two facesfacilitates the assembling of two flow guiding plates 53 in order toform a bipolar plate.

In this example, the flat parts 535 and 536 are placed at one and thesame height and the flow channels 553, 554 and 563 exhibit one and thesame depth with respect to these flat parts 535 and 536.

In the example, the access orifices 551 are intended to communicate withan opening 40 made through the flat part 535. The orifices for access tothe first end of the flow channels 563 are intended to communicate withan opening 43 made through the flat part 535. The openings 40 and 43 areintended to be isolated from one another via seals, in a way known perse. The access orifices 552 are intended to communicate with an opening41 made through the flat part 536. The orifices 562 for access to thesecond end of the flow channels 563 are intended to communicate with anopening 42 made through the flat part 536. The openings 41 and 42 areintended to be isolated from one another via seals, in a way known perse.

FIG. 9 is a view in cross section of an example of a fuel cell 4 usingflow guiding plates 53 as described in detail above.

The fuel cell 4 comprises membrane electrode assemblies 11, 12 and 13.Each membrane electrode assembly comprises in this instance a gasdiffusion layer 21 placed in contact with an anode 111. The anode 111 isattached to a proton exchange membrane 113. A cathode 112 is attached tothe proton exchange membrane 113. A gas diffusion layer 22 is placed incontact with the cathode 112.

The fuel cell 4 additionally comprises flow guiding plates 531, 532 and533 as described in detail with reference to FIGS. 1 to 8. The flowguiding plate 531 forms in this instance, by itself alone, a bipolarplate 51 positioned between the membrane electrode assembly 11 and themembrane electrode assembly 12.

The bottom wall of the flow channels 563 is in this instance in contactwith the gas diffusion layer 21 of the membrane electrode assembly 12.The bottom wall of the flow channels 553 and 554 is in this instance incontact with the gas diffusion layer 22 of the membrane electrodeassembly 11.

The flow guiding plates 532 and 533 form in this instance a bipolarplate 52 positioned between the membrane electrode assembly 12 and themembrane electrode assembly 13. The bottom wall of the flow channels 563of the plate 532 is in this instance placed in contact with the bottomwall 566 of the flow channels 554 of the plate 533. The plate 532 can beattached to the plate 533 via welds (not illustrated) promoting theelectrical conduction between the plates 532 and 533.

Flow circuits 57 are thus formed between the plates 532 and 533 by thecombination:

-   -   of the flow channels 554 of the plate 532 with flow channels 563        of the plate 533;    -   of the flow channels 553 of the plate 532 with flow channels 563        of the plate 533.

The bottom wall of the flow channels 554 of the plate 532 is in thisinstance in contact with the gas diffusion layer 22 of the membraneelectrode assembly 12. The bottom wall of the flow channels 563 of theplate 533 is in this instance in contact with the gas diffusion layer 21of the membrane electrode assembly 13.

It is found that the use of plates 531 to 533 exhibiting the samegeometry makes it possible to form two different types of bipolar plates51 and 52, while guaranteeing:

-   -   identical respective fuel flow rates through these bipolar        plates;    -   identical respective oxidant flow rates through these bipolar        plates.

The geometry of an object is normally defined by the shape of an objector by its morphological characteristics.

The flow channels 563 of the plate 531 are intended to be traversed byoxidant, for example air. The flow channels 553 of the plate 531 areintended to be traversed by fuel, for example molecular hydrogen. Theflow channels 554 are not intended to be traversed by a flow (or else tobe traversed marginally by flow through the diffusion layers) as aresult of the presence of flow restrictions, this being the case despitethere being one and the same cross section of the flow channels 553 and554 over most of their median part. For identical flow conditions, thedrop in pressure through the flow channels of the face 55 is greaterthan the drop in pressure through the flow channels of the face 56.

Thus, it is possible to render uniform the drops in pressure in the fueland oxidant flows, this being the case despite differences in molar flowrates and differences in viscosity of the fluids in these flow channels.Furthermore, the presence of the walls 555 and 556 does notdetrimentally affect the uniformity in the distribution of the fluids atthe face 56.

The flow channels 563 of the plate 532 are intended to be traversed byoxidant. The flow channels 553 of the plate 533 are intended to betraversed by fuel. As in the preceding case, the flow channels 554 ofthe plate 533 are not intended to be traversed by fuel, as a result ofthe presence of flow restrictions.

The drops in pressure for the fuel are thus identical for both types ofbipolar plates 51 and 52. The drops in pressure for the oxidant are alsoidentical for both types of bipolar plates.

In the oxidant flow channels, the gas flow is carried out between aninlet and an outlet of one and the same flow channel 563. The flow isthus in this instance of the parallel type.

In the fuel flow channels, the gas flow is carried out between an inletand an outlet of one and the same flow channel 553. The flow is thus inthis instance of the parallel type.

The bipolar plate formed of the plates 532 and 533 comprises a flowcircuit 57 intended to be traversed by cooling liquid. In order tolighten the bipolar plate formed from the plate 531, this is devoid of acooling liquid flow circuit.

FIGS. 10 to 13 are views in longitudinal section which make it possibleto illustrate different flows of fluids through the bipolar plates 51and 52. The flows in a dotted line correspond to flows of coolingliquid. The flows in a broken line correspond to fuel flows. The flowsin a solid line correspond to oxidant flows.

In order to make possible the formation of bipolar plates 51 having asingle flow guiding plate 53 and of bipolar plates 52 having double flowguiding plates 53, in order to be able to use one and the same geometryof flow guiding plates 53, the latter advantageously exhibit an axis ofsymmetry. The axis of symmetry is typically perpendicular to a medianplane of the plate.

FIG. 14 is a view in perspective of flow channels on the anode side of asecond embodiment of a conducting sheet of a flow guiding plate 53according to the invention. This second embodiment differs from thefirst embodiment in the presence of additional flow restrictions in theflow channels 554. Additional walls 555 are thus built in order to sealthe median part of the flow channels 554.

In this embodiment, two successive flow channels 563 are thus incommunication at their median parts, as a result of the passages formedin order to put in the walls 555 and 556. Consequently, possibledispersions in drops in pressure between two successive channels 563 canbe rendered more uniform by bringing them into communication in such away.

FIG. 15 is a view in perspective of flow channels on the anode side of athird embodiment of a conducting sheet of a flow guiding plate 53according to the invention. This third embodiment differs from thesecond embodiment in the absence of flow restrictions at the ends of theflow channels 554. Flow restrictions are in this instance inserted inthe median part of the flow channels 554. Additional walls 555 are thusbuilt in order to seal the median part of the flow channels 554.

In this embodiment, two successive flow channels 563 are thus incommunication at their median parts, as a result of the passages formedin order to build the walls 555 and 556. Consequently, possibledispersions in drops in pressure between two successive channels 563 areagain rendered uniform by bringing them into communication in such away.

FIG. 16 is a view in section of an alternative form of flow guidingplate 53 at a first end of the flow channels. The flow guiding plate 53differs from that of the alternative form of FIGS. 1 to 8 in thegeometry of the walls 555 and 556. The walls 555 and 556 do not extendas far as the top of their flow channel 554 (or do not extend as far asthe bottom of the interposed flow channels 563). Consequently, a flowcan pass through a flow channel 554 by surmounting the walls 555 and556, but with drops in pressure which are much greater than the drops inpressure through a flow channel 553. Advantageously, the walls 555 and556 extend at least as far as three quarters of the depth of the flowchannels 563 of the face 56.

In the examples illustrated, the flow channels exhibit a straight shapealong the longitudinal direction. It is possible, of course, to provideother flow channel geometries, for example flow channels havingundulations along their longitudinal direction.

1. A flow guiding plate for a fuel cell, comprising: a conducting sheetincluding a first flat part, a second flat part, and flow channelsextending between the first flat part and the second flat part, the flowchannels including flow channels on a first face of the conducting sheetand flow channels on a second face of the conducting sheet, the secondface being opposite to the first face, wherein the flow channels on thefirst face alternate with, and are parallel to, the flow channels on thesecond face such that two successive flow channels on the first face areseparated by walls delimiting a flow channel on the second face, whereinthe flow channels on the first face include a first group of flowchannels and a second group of flow channels, wherein each flow channelof the first group of flow channels on the first face and the flowchannels on the second face include a first access orifice at the firstflat part and a second access orifice at the second flat part, and across section of each of the flow channels between the first accessorifice and the second access orifice is greater than a cross section atthe first access orifice and at the second access orifice, wherein eachflow channel of the second group of flow channels on the first faceincludes a flow restriction at the first flat part and at the secondflat part such that a pressure drop across each flow channel of thesecond group is greater than a pressure drop across each flow channel ofthe first group, a cross section at each of the flow restrictions of theflow channels of the second group being smaller than the cross sectionat the first access orifice and at the second access orifice of the flowchannels of the first group, and wherein the first face comprises analternation of flow channels of the first group and of flow channels ofthe second group.
 2. The flow guiding plate as claimed in claim 1,wherein each flow channel of the second group of flow channels includesa first wall at a first end of the flow channel and a second wall at asecond end of the flow channel, each of the first and second wallsextending from a bottom of a deepest portion of the flow channel to atop of the deepest portion of the flow channel to form the flowrestriction.
 3. The flow guiding plate as claimed in claim 2, whereinthe walls that separate each of the successive flow channels of thefirst face extend to a same height as the first and second walls of thesecond group of flow channels.
 4. The flow guiding plate as claimed inclaim 2, wherein each flow channel of the second group of flow channelsincludes a third wall at the first end of the flow channel and a fourthwall at the second end of the flow channel, the third wall extendingupward from the first flat part and the fourth wall extending upwardfrom the second flat part.
 5. The flow guiding plate as claimed in claim1, further comprising: walls extending upward from the first flat partand the second flat part to form access orifices for the second group offlow channels.
 6. The flow guiding plate as claimed in claim 1, whereineach flow channel of the first group of flow channels includes a firstwall at a first end of the flow channel and a second wall at a secondend of the flow channel, the first wall extending down from the firstflat part to a bottom of a deepest portion of the flow channel to formthe first access orifice, and the second wall extending down from thesecond flat part to the bottom of the deepest portion of the flowchannel to form the second access orifice.
 7. The flow guiding plate asclaimed in claim 1, wherein each flow channel of the second group offlow channels includes a first wall at a first end of the flow channeland a second wall at a second end of the flow channel, each of the firstand second walls extending from a bottom of a deepest portion of theflow channel to a top of the deepest portion of the flow channel to formthe flow restriction, wherein each flow channel of the first group offlow channels includes a first wall at a first end of the flow channeland a second wall at a second end of the flow channel, the first wallextending down from the first flat part to a bottom of a deepest portionof the flow channel to form the first access orifice, and the secondwall extending down from the second flat part to the bottom of thedeepest portion of the flow channel to form the second access orifice,and wherein each of the first and second walls of the second group offlow channels extends higher than each of the first and second walls ofthe first group of flow channels.
 8. The flow guiding plate as claimedin claim 1, wherein each of the flow restrictions extends at least asfar as three quarters of a depth of the flow channels of the secondface.
 9. The flow guiding plate as claimed in claim 1, wherein each ofthe flow restrictions includes a wall extending as far as a bottom ofthe flow channels of the second face.
 10. The flow guiding plate asclaimed in claim 1, wherein the flow channels of the first group and ofthe second group exhibit a same cross section in their median part. 11.The flow guiding plate as claimed in claim 1, wherein the flow channelsof the first group and the flow channels of the second group exhibit asame depth over at least three quarters of their length.
 12. The flowguiding plate as claimed in claim 1, wherein the flow restriction ofeach flow channel of the second group delimits a passage between twoflow channels of the second face.
 13. The flow guiding plate as claimedin claim 1, wherein a shape of the first face of the sheet is acomplement of a shape of the second face of the sheet.
 14. The flowguiding plate as claimed in claim 1, wherein the sheet is a sheet ofsteel alloy, of titanium alloy, of aluminum alloy, of nickel alloy, orof tantalum alloy.
 15. The flow guiding plate as claimed in claim 1,wherein the flow channels of the first and second faces have a samedepth with respect to the first and second flat parts.
 16. The flowguiding plate as claimed in claim 1, wherein the sheet exhibits an axisof symmetry.
 17. A fuel cell comprising: first, second, and thirdguiding plates as claimed in claim 1; first, second, and third membraneelectrode assemblies each comprising a proton exchange membrane, acathode, and an anode which are attached on either side of the protonexchange membrane; first and second gas diffusion layers; the firstguiding plate forming a bipolar plate interposed between the first andsecond membrane electrode assemblies, the first gas diffusion layercovering the flow channels of the first face of the first guiding plateand being in contact with the first face and with the anode of thesecond membrane electrode assembly; the second and third guiding platesdelimiting, between them, a flow circuit and forming a bipolar plateinterposed between the second and third membrane electrode assemblies,the first face of the second guiding plate being in contact with thesecond face of the third guiding plate, the second gas diffusion layercovering the flow channels of the first face and being in contact withthis first face of the third guiding plate and with the anode of thethird membrane electrode assembly.
 18. The fuel cell as claimed in claim17, wherein the first, second, and third guiding plates are guidingplates in which the flow restrictions of the first guiding plate are incontact with the first gas diffusion layer and the flow restrictions ofthe third guiding plate are in contact with the second gas diffusionlayer.
 19. The fuel cell as claimed in claim 17, wherein the first,second, and third guiding plates are traversed by openings communicatingwith the flow circuit.
 20. The fuel cell as claimed in claim 17, whereinthe first, second, and third guiding plates exhibit identicalgeometries.